I suspect during a repair previous owner rewired wrong runs with 12 V attached but only if switch in 24 V position Internal wiring impossible to find but found the attached
1) since using only 12 V only 50% thrust ?
2) 12/24 volt switch on attached only pushes 12 or 24 volts on to on off switch so why are two wires ID out of Rotary switch which must be the feed to one brush
3) so plan to check 12/24 v switch & see that it is wired correctly as a start & working
The diagram attached is the only 12/24 Volt diagram I have found with batteries & internal wiring
1) since using only 12 V only 50% thrust ?
2) 12/24 volt switch on attached only pushes 12 or 24 volts on to on off switch so why are two wires ID out of Rotary switch which must be the feed to one brush
3) so plan to check 12/24 v switch & see that it is wired correctly as a start & working
The diagram attached is the only 12/24 Volt diagram I have found with batteries & internal wiring